Two stroke knocking problems 2

[22350]

I have a Velocette 250cc flat twin two stroke.

As nothing was available for the engine, we made the following modifications:

Wiseco Pistons
Nikasil steel cylinders
Modernized combustion chambers.

We just finished rebuilding it and started jetting.

The bike was very over-jetted and we brought it into line.

We are having a knocking problem. We have to severely over-jet the bike to get the knocking to go away. Like smoke screen over-jetting.

The original compression ratio was 7.5:1. Our current compression is 11:1

The knocking is heat related, but we don't see evidence of detonation on the plugs. Plug color is good, but rich.

We are running AMSOIL AT 32:1, which is less than the manufacturer's 20:1.

The bike does have smallish cooling fins:

<<

http://picasaweb.google.com/oddssodds/ViceroyScott#5438656134821149778>>

Here is the engine design notes:

<<

http://picasaweb.google.com/oddssodds/JerryNotes#>>

Any ideas?

Thanks,

Paul

 

[patprimmer]

To much ignition advance.

To low octane.

Inadequate cooling.

Hot spot in chamber.

To much piston to bore clearance.

To much crank end float.

Insufficient piston to head clearance.

To much dudgeon pin clearance.


Regards
Pat
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[CH5OH]

introduce a thicker cylinder head packing

 

[Tmoose]

What do you get for psi with a compression test?

 

[22350]

We got 155psi

@patprimer

To much ignition advance.

To low octane.

Inadequate cooling. DON'T THINK SO, TEMP ON CYLINDERS IS 252F

Hot spot in chamber. DON'T KNOW

To much piston to bore clearance. NO

To much crank end float. NO

Insufficient piston to head clearance. DON'T THINK SO, LOOK AT THE DESIGN SPECS I POSTED

To much dudgeon pin clearance. NO

 

[22350]

OOPS @patprimer

To much ignition advance. NO, ALMOST NONE

To low octane. NO

 

[Tmoose]

Is this essentially a stock engine using the stock intake and exhaust, and port timing, and ignition timing, but with increased compression and modern cylnder head design?

What kind of seal is used in the crankcase to separate/isolate the cylnders?

Under what operating conditions and throttle position does the knocking occur?
What were you looking for on the plugs as evidence of detonation?
Have the heads been off lately?
What do the undersides of the piston crowns look like? Dark brown varnish, or flaking char, or ??

That's a high compression pressure on a modern (?) air cooled 2-stroke, although the exhaust timing is pretty short. I think it would need an effective squish or quench type combustion chamber to help with detonation. Around 0.030 inch piston to head clearance. What is the current piston-to-head clearance? Also ventilation under the piston via holes would keep things cooler and reduce stagnation.

Head temp with under spark - plug washer sensor is a useful measurement. Perhaps more important than any cylinder measurement. The cylinder by the ex port is a lot hotter than over the transfer ports, and the mixture gets compressed prior to and during firing in a doorway in a neighborhood across town anyhow.

If the exhaust system is restrictive it will cause overheating and detonation. Can happen By design, or if a screen type spark arrestor plugs up with carbon, or .... ..............
Does it use fixed ignition timing?
What was the stock ignition timing?
What is the ignition timing now? ( close to 20 BTDC? )
I'd look at the spark plugs to evaluate timing based on this info -

http://www.strappe.com/plugs.html

I think some folks are concerned that the knocking is possibly mechanical rather than combustion related.

 

[patprimmer]

I spent some time looking at your scribbled notes and could make no sense of them.

Regards
Pat
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[Dan320]

A 200cc flat twin Velocette 2-stroke?
What model is that?

 

[malbeare]

Is it possible that some head gasket is protruding into the combustion chamber edge?.
sometimes if a copper or alluminium gasket is reused several times then it thins out and protrudes causing a hot edge . just my 2 cents
cheers malbeare

 

[Dan320]

If you are trying to run this Velocette 200cc flat twin 4-stroke flathead engine as a 2-stroke I can imagine you will have lots of problems besides knocking.
Please keep us informed on your progress.

 

[benta]

It seems to be the engine from a Velocette Viceroy. Indeed it is a 2-stroke boxer.

http://en.wikipedia.org/wiki/Velocette

Benta.

 

[btrueblood]

What octane ARE you running? Did you try increasing octane, what were the results?

 

[TDIMeister]

The increase in the compression ratio from 7.5 to 11 seems huge in magnitude, especially for a 2-stroke. Are these numbers geometric of effective? Notwithstanding better combustion chamber shaping, an engine of that vintage likely does not have any meaningful measures for scavenging and tuned pipes that are the norm for modern 2-strokes, so I wouldn't be surprised if the knocking comes from reduced combustion velocity due to very high residual gas content.

Sorry to say, I think you should revisit a lower compression ratio, whilst preferably maintaining a squish area. Maybe also try a twin spark plug setup.

 

[TDIMeister]

I committed a typo above. It should have read, "Are these numbers geometric *or* effective?"

 

[TDIMeister]

This is unrelated but I find it nonetheless interesting... if the engine is a boxer-twin 2-stroke (180-degree piston pin offset), then both pistons are simultaneously reaching TDC, making this a parallel-firing twin. This might have been due to the need to accommodate crankcase scavenging.

 

[22350]

ok, I just found out what the problem was. The left cylinder was using a base gasket, which was 10 thou too thin.

That gave me knocking in that cylinder. We are still getting some light knocking, so we are going to drop the compression a little more.

P.

 

[Tmoose]

was the 155 psi on the cylinder with thin base gasket?
Was the knocking detonation, or mechanical contact?

Depending on the other equipment especially complexity of the exhaust system on the engine "raising" the exhaust port would lower the effective compression ratio. For the last few decades 27 degrees of "blowdown" between exhaust and transfer opening has been a pretty desireable number, even on relatively low revving 400 cc CZ MX engine (whose exhasit timing was pretty close to 90 degrees B/ATDC. The first page of notes suggests you have less than 10 degrees. A later page (4) suggests the cylinder was shortened 0.092", yet the transfer timing remained the same ( different piston?) and the ex port was raised about 5 degrees.

 

[ryderKID]

I noticed the same as Tmoose, though you said you used Wiseco pistons and cylinder sleeves, your later measurements didn't reflect that so I was a bit confused. Also as stated already, I usually have had better luck if I remove the spark arrestor and/or decrease exhaust back pressure on small 2 strokes after increasing the compression.

Sounds like a fun project, best of luck!